1. Field of the Invention
The present invention relates to devices for changing the direction of recording medium for use in image forming apparatus such as laser beam printers, copying machines or printing presses.
2. Description of the Related Art
Copying machines having the duplex copying function of forming images on both surface of copy paper or those having the composite copying function of repeatedly forming images on one surface of copy paper have incorporated therein a device for changing the direction of transport of copy paper on completion of the first copying cycle for the paper. The direction changing device (hereinafter referred to as the "switchback device") is adapted to move the copy paper or like recording medium forward and backward, so that the device, which is capable of changing the direction of the recording medium, further has the advantage of eliminating the need to incorporate an elongated bent transport channel into the copying machine or like image forming apparatus for changing the direction of the medium.
Laser printers also include such a switchback device for changing the direction of recording media with the above advantage.
The conventional switchback devices of the type mentioned include those disclosed in Japanese Utility Model Publication No. 62-18594 and Japanese Laid-Open Patent Application No. 62-29359.
The switchback device disclosed in the former comprises three rollers, i.e., a central roller and two rollers arranged on opposite sides of the central roller in contact therewith. The nip of one of the side rollers and the central roller is positioned within an inlet channel, and the nip of the other side roller and the central roller is positioned within an outlet channel. In this arrangement, the recording medium is sent from the inlet channel into a switchback portion ahead of the channel, whereupon the medium is sent backward into the outlet channel. In these channels, the recording medium is transported by the rollers, so that with laser printers of the type wherein the laser scanning station is positioned upstream from the inlet channel with respect to the direction of transport of the medium, there is the problem that the transport of the medium through the scanning station must be timed with the transport thereof by the rollers.
With the switchback device disclosed in the latter, the recording medium is advanced into a switchback portion from an inlet channel and is thereafter sent out into an outlet channel by the elastic force of a spring member. However, the recording medium, when moved by the spring member, will jam the channel if the medium is low in stiffness, i.e., in strength. Further in the case where the device is used for different kinds of recording media having varying lengths, the spring force acting on recording media differs with the length. It is therefore difficult to select a spring member having optimum characteristics.